1. Field of the Invention
The invention relates to driving circuit; in particular, to an output buffer applied in a source driver of a liquid crystal display.
2. Description of the Prior Art
In recent years, with the continuous progress of display technology, various types of display apparatuses, such as a LCD display and a plasma display, are shown in the market. Because the volume of the LCD display is much smaller the conventional CRT display, the LCD display using smaller desk space is convenient for the people in modern life.
In general, the driving apparatus of the TFT-LCD display mainly includes a source driving circuit and a gate driving circuit. For the TFT-LCD display having high quality, high resolution, and low power consumption, the source driving circuit plays a very important role. Please refer to FIG. 1. FIG. 1 illustrates a schematic diagram of a circuit structure of a conventional source driver.
As shown in FIG. 1, the source driver 1 includes a receiver 10, a data register 12, a bi-directional shift register 14, a line latch 16, a level shifter 18, a digital-to-analog converter 20, and an output buffer 22. The main power domain of the source driver 1 includes digital power sources VDD, VSS and analog power sources AVDD, HAVDD, AGND. In general, a plurality of output channels Y1˜Yn of the source driver 1 can be divided into the even output channels and the odd output channels. If the output voltages of the even output channels are higher than HAVDD, the output voltages of the add output channels will be lower than HAVDD, and vice versa. This half analog power source structure of the TFT-LCD display can achieve the effect of saving system power consumption.
The output buffer 22 of the source driver 1 is usually realized by an operational amplifier (OPAMP). For the source driver 1 of the TFT-LCD display, its visual performance is determined by the operational amplifier, especially the slew rate which is important to the TFT-LCD display applied in the high frame rate application.
Please refer to FIG. 2. FIG. 2 illustrates a circuit diagram of a conventional operational amplifier POS with positive polarity operated under a half analog power source. As shown in FIG. 2, the circuit of the operational amplifier POS includes a differential input stage DIS, bias current sources IBSP and IBSN, an output stage OS, a compensation capacitor Cc, and a comparator CP. Because the operational amplifier POS with positive polarity is operated under the half analog power source, its input signal Vin—POS, output signal Vout—POS, and output stage OS have operation voltages between AVDD and HAVDD, and the comparator CP has an operation voltage between AVDD and AGND. And, the comparator CP can be realized by a cascading source follower, as shown in FIG. 3.
As shown in FIG. 3, because the swing level of the input signal and output signal, the NMOS transistor MN1 will suffer body effect more serious than the PMOS transistor MP1 and its threshold voltage will become larger. Therefore, the NMOS source follower will become less sensitive, and the response of the control signal Ctrl_P will be slower than the control signal Ctrl_N to cause the slew rate of output driving and sinking become unbalanced, and the visual performance of the TFT-LCD display will be seriously affected.
Therefore, the invention provides an output buffer applied in a source driver of a liquid crystal display to solve the above-mentioned problems occurred in the prior arts.